Much of the pre-election alliance between Donald Trump and Vladimir Putin is hidden in plain view. We know that Putin resented Obama’s Russia policy and feared a harder-line Hillary Clinton presidency even more. We know that Putin deliberately engaged in cyber-warfare to embarrass the Clinton campaign. We know that Trump—loudly and publicly—urged the Russians to keep leaking.

We also know that Trump and his family—a single commercial entity—had extensive business relationships with Russia. We know that Trump campaign officials had repeated contacts with senior Russian officials, both during the campaign and in the interregnum between the election and Trump’s inauguration. We know that Trump has been extremely flattering in his descriptions of Putin, and is drastically changing American foreign policy to back away from the Atlantic Alliance and give Russia a freer hand in Europe and elsewhere.

None of this is quite enough to convict Trump of violating the Constitution, either for outright treason or for personally profiting from Putin’s favors, which transgresses both the Emoluments Clause and statutory law.

So what would be sufficient?

One thing would be evidence of an explicit deal. We know that intensive investigations by the FBI and CIA, with help from the NSA, are continuing, to determine the exact extent and specificity of contacts and mutual commitments between the Trump campaign and Putin. It obviously enrages Trump, as an autocratic CEO accustomed to total power, that he can’t turn these off; and that the more he tries, the more leaks he invites.

But it will not be sufficient for these investigations to document more winks and nods and tacit bargains. What’s needed is some kind of smoking gun, in the form of an explicit quid pro quo.

The trouble is that deals like this are seldom put in writing, and that even transcripts of conversations rarely reveal explicit quid pro quos. Even if some of Trump’s campaign surrogates, such as Michael Flynn, made deals, Trump could say that they were freelancing and that this was done without his knowledge. We will not know just how explicit these deals were until the investigations are wrapped up.

Another possibility that could put Trump’s presidency at risk is a bungled cover-up.

It may be that Trump digs himself a bigger hole by trying to suppress the investigations, as he came close to doing when White House aides tried to enlist the FBI to rebut an accurate New York Times story about pending investigations.

But there is one other potential set of revelations that could endanger Trump’s presidency: As Deep Throat didn’t quite say, except in the movie version of All the President’s Men, “Follow the money.”

Follow the money. Trump’s entire career has been about money—making as much money as possible, often in smarmy ways.

So the other set of revelations that Trump must be fearing are disclosures that the Russians either put money into Trump’s pocket or found ways to put money into his campaign. This also has to be a focus of ongoing investigation.

The Russians have been helping to finance the campaign of the French ultra-nationalist Marine LePen, whose revisionist views on Russia and flattering comments on Putin are eerily reminiscent of Trump’s.

The Russians, famously, not only use disinformation and disruption and attempted blackmail. They use money.

No wonder Trump is coming unglued, worrying that Obama bugged his office. The security agencies didn’t need to bug Trump. They bugged the Russians. If they stumbled on Americans doing political deals with the Russians, it was by accident.